1. Field of the Invention
The present invention relates generally to an electro-optical measuring system, and more particularly to a system which employs a laser or light beam to scan the object whose dimension is to be measured, to provide highly precise measurements.
2. The Prior Art
For the accurate measurement of the diameter, position, or thickness of soft, delicate, hot, or moving objects, non-contacting measuring systems must be used. Prior-art devices of this character include capacitive gauges, eddy-current gauges, air gauges, gamma and X-ray gauges, and optical gauges. Only the optical and nuclear gauges can work at distances greater than a small fraction of an inch with sufficient sensitivity. The nuclear gauges permit large working distances; however, they are extremely expensive and susceptible to systematic errors due to slight variations in the chemical composition of the object being measured.
Optical gauges have advantages because of the nature of light itself. The principal advantages are:
1. They do not require direct mechanical contact between the gauge and the object to be measured; PA1 2. The distance from the gauge to the object to be measured can be large; PA1 3. Light variations are directly convertible to electrical signals; PA1 4. The response time is limited to that of the photosensor and its electronics; PA1 5. The measurements are independent of the chemical composition of the object; and PA1 6. Measurements can be made at a rapid rate, e.g. 5-10x faster than traditional gauging devices.
Prior-art optical gauges employ various techniques to produce a scanned light beam, to measure the location of the edge of an object, and to measure the time interval between the occurrence of two sensed edges.
Such gauges have been available with accuracies of the order of 2.5-12.5 micrometers, depending on the size of the object being measured. For example, Petrohilos U.S. Pat. No. 3,905,705 issued Sept. 16, 1975 discloses an optical measuring apparatus wherein the size of an object is measured by counting the number of constant frequency pulses which occur between two edge pulses. This method yields a measurement precision of approximately 12.5-25 micrometers for a range of object dimensions up to 50 millimeters, although it is more accurate with objects of small diameters, such as optical fibers.
Altman, et al., U.S. Pat. No. 4,168,126 issued Sept. 18, 1979 discloses an optical measuring apparatus which produces an extremely linearly scanned light beam. However, when objects with diameters of 10 millimeters or more are being measured, fluctuations occur in the measurement. These fluctuations follow a 1/f noise spectrum so that averaging does not significantly reduce them, thus limiting the possible accuracy of the system to the order of 2.5-5 micrometers when measuring a 50 millimeter object.
The assignee of the instant invention has been marketing a laser measuring system developed by coworkers of the applicants herein which uses a collimated scanned beam instead of the focused beams used by Petrohilos and Altman et al., obtaining accuracies of 2.5-5 micrometers with objects of the order of 100 millimeters in size, by using the exact edge sensing technique disclosed in the Zanoni U.S. Pat. No. 3,907,439, issued Sept. 23, 1975, along with other improvements to overcome systematic errors.
Other scanned beam systems have also been constructed with rotating mirrors, prisms and gratings operating in conjunction with optical lenses and/or mirrors to produce reasonably precise measurements, i.e., 2.5-25 micrometers.
While these prior-art techniques are useful for some measurements, they cannot be used for exceptionally precise measurements required in many industrial operations. For example, in the metal fabrication industry many parts have dimensions with tolerances of 2.5-5 micrometers. The "gauge makers" rule requires that the measuring apparatus has a precision 1/10 this tolerance i.e., 0.25-0.5 micrometers.